I have just upgraded to a quad core and installed vista64 it seems stable and drivers are fine.

as a rule of thumb i would say vista64 and xp32

There is nothing wrong with XP64 except you will probably see the new drivers dwindle away fairly rapidly for items that are outside the scope of the business market.

Vista32, what is the point your performance will probably be better on XP32

If you want to use all of that 4Gb you need to go 64bit BUT remember that if you can see 3.5Gb in 32 bit you are not actually going to gain much as that will be eaten by the additonal memory requirements of 64Bit processes

for your system, take your pick, go with your preference and you will be fine